1. Field of the Invention
The present disclosure relates to an image forming apparatus that reduces color misregistration when images of a plurality of color components are superimposed to form a color image.
2. Description of the Related Art
An image forming apparatus employing the electrophotographic process includes image forming units that form toner images of different colors for respective color components. The toner images formed by the image forming units for respective color components are transferred onto an intermediate transfer member to be superimposed. The toner images, after having been transferred onto a recording material, are fixed onto a recording material as a full-color image by heat and pressure of a fixing device.
In the image forming apparatus, when the images for the respective color components have been transferred onto the intermediate transfer member, if there is a positional deviation between the images for the respective color components, color misregistration will be produced in an image on the recording material. Therefore, an image forming apparatus discussed in U.S. Patent Publication No. 2011/0280633A1, forms a measurement image for color misregistration detection on an intermediate transfer member, and adjusts a position and a timing at which the image is formed for each image forming unit, based on a result of detection of the measurement image for color misregistration detection.
In the image forming apparatus discussed in U.S. Patent Publication No. 2011/0280633A1, a first image forming unit forms a first measurement image having a first color onto the intermediate transfer member, and a second image forming unit forms a second measurement image having a second color at a position on the intermediate transfer member away from the first measurement image by a predetermined distance. The first measurement image and the second measurement image on the intermediate transfer member pass through a measurement position, and thereby a positional difference between the first measurement image and the second measurement image is detected. The measurement position is a position on the intermediate transfer member at which an optical sensor radiates light. In U.S. Patent Publication No. 2011/0280633A1, a position at which the second image having the second color is formed by the second image forming unit is adjusted based on the positional difference thereof. Accordingly, color misregistration of an image, which is formed by transferring the second image having the second color to be superimposed on the first image having the first color, is reduced.
The optical sensor outputs a signal determined according to an amount of received light, by receiving the light reflected by the intermediate transfer member or the measurement image for color misregistration detection. The positional difference is determined according to a time difference between a time period during which an output value when the optical sensor has received a reflected light from the first measurement image exceeds a threshold value, and a time period during which an output value when the optical sensor has received a reflected light from the second measurement image exceeds the threshold value.
Further, an achromatic color toner and the intermediate transfer member have a low reflectance, and their difference is small. Therefore, a position of an image formed with the achromatic color toner on the intermediate transfer member cannot be detected by the optical sensor. Thus, in U.S. Patent Publication No. 2011/0280633A1, a position of the image formed with the achromatic color toner on the intermediate transfer member is detected using a composite pattern. The composite pattern is images obtained by transferring a plurality of images formed with the achromatic color toner separated from each other by a predetermined distance, on an image formed with chromatic color toner. In other words, the composite pattern is a pattern in which an image region formed with the chromatic color toner is exposed from a region between a plurality of images formed with achromatic color toner. The optical sensor can detect a reflected light from the image region formed with the chromatic color toner exposed from the region between the plurality of images formed with the achromatic color toner.
FIG. 11 is a schematic diagram illustrating a reference pattern and a composite pattern formed on the intermediate transfer member in a case where a positional deviation has not occurred, and a reference pattern and a composite pattern formed on the intermediate transfer member in a case where a positional deviation has occurred. The reference pattern is a first measurement image having a magenta toner. The composite pattern is formed by superimposing the second measurement image having a black toner on the first measurement image having the magenta toner Further, a distance Lo between the reference pattern and the magenta image that constitutes the composite pattern does not cause an error, since the magenta image is formed by the same image forming unit for magenta.
In a case where the image formed with the magenta toner and the image formed with the black toner do not have a positional deviation, a positional difference from the reference pattern to the magenta image region exposed from a region between the plurality of images formed with the black toner becomes Lma (target distance). On the other hand, in a case where the image formed with the magenta toner and the image formed with the black toner have a positional deviation, a positional difference from the reference pattern to the magenta image region exposed from the region between the plurality of images formed with the black toner becomes Lmb. In other words, in a case where the image formed with the black toner has a positional deviation, a positional difference Lmb from the reference pattern to the magenta image region exposed from the region between the plurality of images formed with the black toner differs by a positional deviation amount ΔL relative to the target distance Lma. The image forming apparatus discussed in U.S. Patent Publication No. 2011/0280633A1 adjusts a position at which the black image is formed based on the positional deviation amount ΔL, thereby reducing color misregistration of an image formed by transferring the black image overlapping the magenta image.
However, in the composite pattern composed of the chromatic color toner and the achromatic color toner, an amount of the achromatic color toner overlapped on a region formed with the chromatic color toner may be decreased by an influence of temperature or humidity. When an amount of the achromatic color toner overlapped on a region formed with the chromatic color toner decreases, light radiated from the optical sensor penetrates through the achromatic color toner region and is reflected from the chromatic color toner region, thereby increasing an amount of light received by the optical sensor.
As a result, if an output value output from the optical sensor becomes a threshold value or greater, by the optical sensor receiving the reflected light from the chromatic color toner covered by the achromatic color toner, there is a problem that a positional difference between the reference pattern and the composite pattern may be erroneously detected. Consequently, even if a position at which an image is to be formed is adjusted, based on a positional difference between the reference pattern and the composite pattern formed on the intermediate transfer member, there is a problem that color misregistration of an image formed by superimposing the image formed the achromatic color toner on the image formed with the chromatic color toner cannot be inhibited